Abstract

Owing to its large dipole, astrophysicists use carbon monosulfide (CS) as a tracer of molecular gas in the interstellar medium, often in regions where H2 is the most abundant collider. Predictions of the rovibrational energy levels of the weakly bound complex CS-H2 (not yet observed) and also of rate coefficients for rotational transitions of CS in collision with H2 should help to interpret the observed spectra. This paper deals with the first goal, i.e., the calculation of the rovibrational energy levels. A new four-dimensional intermolecular potential energy surface for the H2-CS complex is presented. Ab initiopotential energy calculations were carried out at the coupled-cluster level with single and double excitations and a perturbative treatment of triple excitations, using a quadruple-zeta basis set and midbond functions. The potential energy surface was obtained by an analytic fit of the ab initio data. The equilibrium structure of the H2-CS complex is found to be linear with the carbon pointing toward H2 at the intermolecular separation of 8.6 ao. The corresponding well depth is −173 cm−1. The potential was used to calculate the rovibrational energy levels of the para-H2-CS and ortho-H2-CS complexes. The present work provides the first theoretical predictions of these levels. The calculated dissociation energies are found to be 35.9 cm−1 and 49.9 cm−1, respectively, for the para and ortho complexes. The second virial coefficient for the H2-CS pair has also been calculated for a large range of temperature. These results could be used to assign future experimental spectra and to check the accuracy of the potential energy surface.